1. Field of the Invention
The present invention relates to a MOS-type capacitor suitable for an oscillation circuit which is used in, for example, electronic equipment and which utilizes a resonator such as a quartz resonator or a ceramic resonator, and whose oscillation frequency can be controlled through application of voltage thereto (hereinafter referred to as a VCO (Voltage Controlled Oscillator)), as well as to a semiconductor integrated circuit device comprising a VCO whose oscillation frequency can be changed within a widened range through application of a control voltage to thereby enable fine adjustment of electronic equipment and which can be produced at a reduced cost.
2. Description of the Related Art
FIG. 8 is a graph showing the temperature drift of an oscillation frequency of a crystal oscillator utilizing a typical AT-cut quartz. The vertical axis represents temperature drift .DELTA.f/f.sub.0 (ppm), where f.sub.0 is an oscillation frequency at 25.degree. C., and the horizontal axis represents temperature (.degree.C.). As is apparent from the graph, the oscillation frequency changes within a range of .+-.40 ppm when the temperature changes from -40.degree. C. to +90.degree. C. Further, AT-cut quartz resonators mass-produced under the same conditions deviate from each other in oscillation frequency by as much as 30 to 50 ppm. If the oscillation frequency of electronic equipment, especially a cellular phone or portable information terminal which uses radio waves, deviates from a target frequency due to the above-described causes, various problems occur. Accordingly, the oscillation frequency must be adjusted such that the deviation comes within a range of .+-.10 ppm, more preferably .+-.1 ppm. For such a purpose, a VCO capable of changing its oscillation frequency in accordance with an applied control voltage is used.
FIG. 9 is a circuit diagram of a typical VCO utilizing a quartz resonator or a ceramic resonator. The VCO has external connection terminals 121 and 122 to which a quartz resonator 110 is connected. Also, the VCO comprises a CMOS inverter 123, which constitutes an amplifier in cooperation with a bias resistor (Rf) 126 connected between the input node 124 and the output node 125 of the CMOS inverter 123. A resistor (Rd) 127 is connected between the output node 125 and the external connection terminal 122. Although the resistor 127 is frequently omitted in the case of oscillation circuits of 1 MHz or higher, provision of the resistor 127 is recommended from the viewpoint of the stability of the oscillation frequency. A capacitor (Cd) 128 is connected to the external connection terminal 122. Meanwhile, a capacitor (Cg) 131, a PN-junction capacitor (having the same configuration as that of a PN-junction diode) serving as a variable capacitance element (Di) 132, and a resistor (R1) 133 are connected to the external connection terminal 121 via a capacitor (Cp) 130 for shutting off DC voltage. The other terminal of the resistor (R1) 133 is connected to a Vc terminal 134. An external capacitor for adjustment may be connected to the external connection terminal 122.
The resistor (RD) 127, the quartz resonator 110, the capacitor (Cd) 128, the capacitor (Cg) 131, the PN-junction capacitor serving as a variable capacitance element (Di) 132, and the capacitor (Cp) 130 constitute a resonance circuit, which is driven by the amplifier formed of the CMOS inverter 123 and the bias resistor (Rf) 126. Further, the output from the resonance circuit is fed back to input terminal 124 of the amplifier via the external connection terminal 1221, to which is connected one terminal of the quartz resonator 110 opposite the terminal connected to the output side of the amplifier. Further, a frequency control voltage is input from the Vc terminal 134 to the PN-junction capacitor (Di) 132 via the resister (R1) 133.
In such a circuit configuration, the synthetic capacitance formed of the capacitor (Cd) 128, the capacitor (Cg) 131, the PN-junction capacitor (Di) 132, and the DC-cut capacitor (Cp) 130 of the resonance circuit determine the oscillation frequency f.sub.0. Accordingly, when the capacitance of the PN-junction capacitor (Di) 132 is changed by means of voltage input from the Vc terminal 134, the oscillation frequency f.sub.0 changes.
A curve 72 shown in FIG. 4 represents the voltage-capacitance characteristic (C-V characteristic) of the PN-junction capacitor (Di) 132. In FIG. 4, the horizontal axis represents control voltage, and the vertical axis represents capacitance. When the control voltage is changed within a range of 0 to 4 volts, the capacitance per unit area becomes about one half.
As described above, in the case where a PN-junction capacitor is used as a variable capacitance element, the capacitance becomes about one half upon increase of the control voltage from 0 to about 4 volts. In the case of a quartz resonator being used, variation in oscillation frequency .DELTA.f/f.sub.0 caused by such a capacitance change becomes about .+-.80 ppm.
By contrast, deviation in oscillation frequency is caused by not only temperature drift and production variation among resonators, but also several other factors. Therefore, in many cases, a tuning amount as large as .+-.80 ppm is insufficient, and a variation amount of .+-.100 to .+-.200 ppm is required. Even in the case of a PN-junction capacitor, the capacitance variation ratio can be increased when the concentration of impurities in the vicinity of the PN-junction is controlled to have a certain profile. However, there are many difficulties when such a PN-junction capacitor is formed on the same semiconductor substrate as that of a MOS circuit, CMOS circuit, or the like which constitutes, for example, an amplifier.
Although a scheme in which a plurality of PN-junction capacitors are selectively used through switching may be employed in order to increase the correction amount, this results in an increase in chip size and an increase in complexity of the tuning system.
Another drawback involved in the PN-junction capacitor is loss of oscillation frequency stability which occurs when the DC control voltage applied to the Vc terminal 134 is near 0 V. That is, when the oscillation amplitude of the oscillator exceeds 0.6 V in a state in which the DC control voltage applied to the Vc terminal 134 is near 0 V, forward current flows through the PN-junction capacitor, because the PN-junction capacitor has the same configuration as that of a diode. In this case, the oscillation frequency becomes unstable.
Meanwhile, a MOS-type capacitor has been used as a variable capacitance element. The MOS-type capacitor has a structure as shown in FIG. 10.
FIG. 10 shows a schematic cross-sectional view of such a MOS-type capacitor. A polysilicon gate electrode 153--which forms a MOS capacitor--is provided on a P.sup.- -type semiconductor substrate 151 via an insulation film 154.
In such a MOS-type capacitor, when a positive voltage is applied to the gate electrode 153, a depletion layer 155 is formed within the P.sup.- -type semiconductor substrate 151 in the vicinity of the surface thereof. When the positive voltage applied to the gate electrode 153 is increased, a strong inversion layer is formed at the surface of the substrate, so that an increase in the thickness of the depletion layer saturates or stops regardless of the applied voltage.
The capacitance C of the MOS-type capacitor is the series capacitance of the capacitance Co of the insulation film 154 and the capacitance of the depletion layer 155. Accordingly, although the synthetic capacitance initially decreases as the voltage applied to the gate electrode 153 increases, the decrease in capacitance saturates when a strong inversion layer is formed.